Automatic radio direction finding system



Oct. 25, 1949. v c. w. EARP ETAL 2,485,578

AUTOMATIC RADIO DIRECTION FINDING SYSTEM 5 Sheets-Sheet l RP AMP F Filed March 22, 1945 Inventor QHangEs \Jmnmn ERRV F swmm @Emus was TuN Oct. 25, 1949. c. w. EARP ETAL AUTOMATIC RADIO DIRECTION FINDING SYSTEM 3 Sheets-Sheet 2 Filed March 22, 1945 '0 A and/PR F/GG Inventor (.unRLEa \nlmunfl ERR? Oct. 25, 1949. c. w. EARP ETAL AUTOMATIC RADIO DIRECTION FINDING SYSTEM 3 Sheets-Sheet 5 Filed March 22, 1945 mnmwmms WESTON y Attor ey Patented Oct. 25, 1949 2,485,578 AUTOMATIC RADIO DIRECTION FINDING SYSTEM Charles William Earp and Jeffrey Dennis Weston,

London, England, assignors, by mesne assignments,

to International Standard Electric Corporation, New Yorlr, N. Y., a corporation of Delaware Application March 22, 1945, Serial No. 584,150 In Great Britain February 3, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires February 3, 1964 17 Claims. (Cl. 343-121) The present invention relates to an electric wave modulating device employing earthed grid valves, adapted particularly to serve as an electronic switch with special application to a radio direction finding system.

Valves with earthed grids arranged as inverted amplifiers are particularly suited for switching of very high frequencies, as will be explained more fully later on in this specification. A reversing or change-over switch is required in one well known radio direction makes use of the normally provided system of radio communication between the pilot of an aircraft and a ground station, so that the carrier waves radiated by the ground system furnish the required indication. The principal object of the present'invention is to provide a suitable electronic reversing switch which can be used for switching very high switch is fundamentally an inverted 'am adapted in a particular manner to servasmodulator and may be used as a modulator in stead of as a switch without fication.

It will be understood that an inverted amplifier is one in which the control grids of the valves are effectively earthed for the waves to be amlified, the input voltages being applied to the cathodes.

According to the invention, there is provided an electric wave modulating arrangement comprising a thermionic valve having a cathode, a control grid and an anode, means for applying a high frequency alternating voltage wave between the said cathode and earth, means for maintaining the said control grid substantially at earth potential for the high frequency waves, means for applying a low frequency modulating signal voltage between the said control grid and earth, and means for deriving modulated high frequency waves from the said anode.

The invention also provides a balanced electric wave modulating arrangement comprising two thermionic valves each having a cathode, a con trol grid and an anode, means for applying equal high frequency alternating voltage waves respectively in opposite phase between the said cathodes and earth, means for maintaining both the said control grids substantially at earth potential for the high frequency waves, means for finding system which frequencies, which has hitherto presented considerable difllculties The any significant modian alternating voltage at a given frequency beapplying equal low frequency modulating signal voltages respectively in opposite phase between the said control grids and earth, and means for deriving modulated high frequency waves from the said anodes.

According to another aspect, the invention consists in an electronic switch for electric waves comprising a thermionic valve having a cathode, a control grid and an anode, means for intermittently blocking the said valve, means for applying tween the said cathode and earth, means for maintaining the said control grid substantially at earth potential for waves at the given frequency, and means for deriving output waves from the said anode.

The invention finally provides an electronic switch for electric waves comprising two thermionic valves each having a cathode, a control grid and an anode, means for intermittently blocking each. of the said valves in alternate periods, means for applying alternating voltages at a given fre- 20 quency between the said cathodes and earth,

means for maintaining both the said control grids substantially at earth potential for waves at the given frequency, and means for deriving output waves from the said anodes.

The invention will be described in its application'to a particular direction finding system of the kind already mentioned, and is illustrated in the accompanying drawings in which:

Fig. 1 shows a circuit diagram, partly in block schematic form, of the direction finding system;

Figs. 2 and 3 show explanatory diagrams;

Fig. 4 shows a schematic circuit diagram of an electronic switch according to the invention;

Fig. 5 shows a schematic circuit diagram of a combined electronic switch and discriminator according to the invention; and

Fig. 6 shows a preferred form of the electronic switch designed for very high frequencies.

It will be understood that the electronic modulator or switch according to the invention may have applications in systems quite different from that which will be described for illustration.

Fig. 1 shows partly in block schematic form an arrangement which is a particular case of the general system covered by United States specification No. 2,213,273. It is a system suitable for an aircraft for obtaining a direction indication from the waves used for the radio communication channel. It will be assumed for clearness that the wavelength is of the order of cm. and that the waves are vertically polarised.

The aircraft is provided with three straight vertical aerials A1, A: and As each a quarter wavelength long. A1 is the aerial which will normally be used for the communication channel and is arranged midway between the other two aerials A2 and A3, which will be spaced about 30 cm. apart for example. This distance is not critical but should preferably be less than half the wavelength. The aerial A1 is connected to the radio receiver RR through a transmission line or other suitable network N producing a phase change of 'rr/ 2. The aerials A2 and A: are connected together by an impedance Z having the centre point connected to ground. Two other tapping points symmetrically placed in either side of the centre point are connected to a periodically operated switch S adapted to connect these tapping points alternately through a resistance R to the radio receiver. The switch S is controlled by an oscillator G and is shown as a polar relay having a set of change-over contacts I and 2-. It W111 be understood that this is only intended to be diagrammatic, and while the switch could in principle take this simple form, according to this invention an electronic switch to be presently described is employed.

The input to the radio receiver RR. will thus comprise a high frequency wave modulated at the frequency f of the oscillator G. This frequency should preferably be towards the upper end of the audio frequency range where it will not appreciabl efiect the intelligibility of the speech channel, and a frequency of 5 kc. has been found suitable. The demodulated output of the receiver is applied through an amplifier AMP to a tuned circuit or filter F adapted to select the frequency j and to pass it to a discriminating circuit D which is also supplied directly from the output of the oscillator G. The normal speech output of the receiver RR is directed to the operator's receivers OR in the usual way. The pilot will thus hear a 5 kc. tone superposed on the speech and provided that it is not too loud it does not interiere with the speech channel, and its presence has been found to be beneficial since it indicates that the channel is in working order when no speech signals are present. This tone could if desired be eliminated from the speech output by suitable filtering.

It will be understood that the elements shown as blocks without details may take any conventional form, and may be arranged in any convenient manner producing the same result. Thus, for example, the elements AMP and F may be interchanged or combined in one or the amplifier could be omitted if unnecessary.

Having described ingeneral terms the receiving arrangements on the aircraft, the manner in which they operate will now be explained.

Referring to Fig. 2, let (2 be the distance between the aerials A2 and A3. Let A1? he the direc tion of flight of the aircraft. Then the aerials should be placed so that A2 A1 A3 is perpendicular to AiP. Let AiQ be the direction to the transmitting station, and let the angle PA1Q be .9.

Let E cos wt represent the voltage induced by the incoming waves in the aerial A1. Then the voltages induced in A2 and 'As will respectively be E cos (wt-W2) and E cos (qJf+ p/2), where q) is the phase difference of the waves arriving at A2 and As. It is evident that 90: (21rd sin w/A, where A is the wavelength.

Now it is evident from Fig. 1 that the current through the impedance Z is proportional to the difference beween the voltages induced in the aerials A2 and As, that is, it is proportional to E (sin (Ii/2) .sin wt. Thus the voltage applied to the radio receiver through resistance R5 will be proportionai to i E (sin o/2).sin wt, according to the position of the switch 8.

Since the aerial A1 is connected to the radio receiver through the network N, the voltage derived therefrom will be proportional to E sin wt. Hence the total voltage V applied to the radio receiver will be E sin wt (a:b sin gv/2) according to the position or the switch S, where a and b are constants determined by the attenuation or gain of the connecting paths between the aerials and the receiver.

It will be seen that the purpose of the network N is to bring the voltage derived from the aerial A1 into the same phase as the difference between the voltages derived from A: and A: (or into the opposite phase). In practice, of course, all the aerials are commonly connected to the radio receiver through transmission lines which will generally introduce large changes of phase. The function of the network N will therefore bemost easily carried out by making the lines leading to A2 and A: equal in length, and the line leading to A1 longer or shorter by a quarter wavelength.

The two values of the voltage V applied to the radio receiver corresponding to the two positions of the switch S afford the means for determining the phase angle q) in sign and magnitude, and thus from the relation (21rd sin 0) the angle 0 is also determined in sign and magnitude. Thus a left and right indication is provided for the pilot, togetherwith a measure of the angular deviation 0.

These indications are obtained in the following manner:

Referring to Fig. 3, curve a represents the sinewave output of the generator G and curve 1) indicates .when the contacts 8 and 2 of the switch S are closed. During the periods i, 3, 6, etc. the amplitude of the modulated wave supplied to the radio receiver is E (a-i-b sin /2) and during the periods 2, s, 6, etc. it is E (a-b sin -p/2). If qv is positive then curve 0 shows the modulated wave, in which the amplitude is greater in the odd numbered periods. If go is negative, the modulated wave is as shown by curve d, and the amplitude is greater during the even numbered periods. The corresponding demodulated outlines obtained at the output of the radio receiver are shown respectively by curves e and f, and the sine waves passed by the filter F are shown by the curves g and h. It will be seen that g is in the same phase as a while Pi is in the opposite phase. Further, it will beevident that the amplitude of the curves g and it will be proportional to sin i /Z. Referring again to Fig. i, the waves 9' or it oi Fig. 3 are applied to the discriminator D and are compared with the waves a supplied from the generator f. The discriminator comprises a bridge circuit having two arms respectively made up of the similar rectifiers X1 and X2 and the equal resistances R1 and R2. The other two arms comprise the equal resistances Rs and Re and the two similar secondary windings S1 and S2 of a transformer T1, the primary windings P1 and P2 of which are connected to the oscillator G. The secondary windings are connected so that aiding voltages are obtained in the series circuit S1, R3. B4, S2. The horizontal diagonal of the bridge 18 occupied by the secondary winding of the transformer T2 of which the primary winding is connected to the filter F. A direct current indicator I is connected across the resistances R3 and R4.

Let is first be assumed that the aircraft is flying directly towards the ground transmitter.

Then 0 and (p are both zero, and there will be the transformer T2.

some no output from the filter F. Waves a will however be applied for the oscillator G. Suppose that positive half waves generate voltages in the windings Si and S: such that the upper terminals are positive to the lower, as-indicated. Then the rectifier X: will be non-conducting, and a current will fiow upwards through Ba. and practically no current through R4. For negative half waves the other rectifier is non-conducting, and an equal currentwill flow. downwards through R4, so that the net eflect on the indicator I is zero over a number of cycles. Now suppose that the aircraft is fiying so that a positive value of is obtained, then an output will be obtained through the transformer T2. It will be supposed that the transformer T: has been poled so'that during the positive half waves a the junction pointof R: and R4 is made positive as indicated, when 0 is positive. Then during the positive half waves of a. the rectifier X: is non-conducting as before, and the current through Ra will be upwards and will be proportional to the sum of the voltages generated in one half of the transformer T1 and in During the negative half waves of (1, X1. is nonconducting and the current through R4 is now downwards but is proportional tothe difierence between the above-mentioned voltages. Thus the net eifect on the instrument I over a number of cycles will be that of an upward current through the resistances R: and R4 proportional to the voltages generated in Ta.

If the value of 0 is negative, the waves )1. will be applied through the transformer T2 instead of g and the secondary voltage will be reversed, producing a downward reading on the indicator.

It should be noted that the resistances R1 and R: are relatively high resistances used to swamp variations in the characteristics of the rectifiers X1 and X2, whereby a better balance of the circult is maintained. The resistances R3 and R4 may be made up of a potentiometer with adjustable centre tap to enablea zero balance to be easily set.

The form of the switch S according to this invention is an electronic switch using a pair of valves arranged with their control grids at ground potential for the high frequency waves, the bias of the grids being controlled by the oscillator G in such a manner that the valves are alternately blocked. The arrangement is shown in Fig. 4. The two valves V1 and V: are arranged in the manner of a balanced modulator. The impedance Z to which the antennas Az-and A3 are connected is shown as a balanced transformer with the ends of its secondary winding connected respectively to the cathodes of the two valves. The control grids are connectedthrough resistances R1 and Re to the secondary winding of a transformer T: which has its centre tap connected to ground. The primary winding T3 is connected to the output of the oscillator G. The two anodes are connected together and to the high tension supply through a resonant output circuit tuned to the frequency of the incoming waves. A tapping on the inductance L is connected through the resistance R5 to the aerial A1 and receiver RR as in Fig. 1. A cathode bias network Q may be connected if necessary between the centre tap of the secondary winding of Z and ground. The control grids are connected to ground through by-pass condensers K1 and K2 having negligible reactance at the frequency of the incoming waves. A similar by-pass condenser K2 is connected across the terminals of the high tension source.

ill

. 6 a The control grids may also be connected to ground through corresponding rectifiers KI and X4 directed so as to conduct when the gridstend to become positive.

The amplitude of the waves at frequency 1 generated by the oscillator G should be relatively large so that the potential of the control grids is sharply driven beyond the cut oif point by the negative loops of the waves applied thereto. It will be apparent therefore that the valves V1 and V2 will be blocked in turn by alternate half waves. The rectifiers X1 and X4 act as limiters to prevent the grids from being driven positive by the positive half waves, so that in fact the controlgrid-cathode voltage of each valve will alternate between a small negative value (depending on the bias produced by the network Q) and a large negative value beyond the cut off voltage. If the amplitude of the waves applied from G is suificiently large, the two valves will behave substantially like the simple diagrammatic switch S shown in Fig. 1.

A certain amount of grid current limitation of the positive grid voltage will be obtained by the resistances R1 and Rs without the help of the rectifiers X1 and X4, but the rectifiers should be used if grid current is undesirable.

It will be noted that Fig. 4 is really a balanced modulator employing valves with grounded grids. If, for example, the rectifiers IQ and x4 are omitted and if G is replaced by a source of modulating signals, and if A: and A3 are replaced by a carrier wave source, modulated waves will be obtained from the resonant circuit LC, provided, of course, that the valves are appropriately biassed. By omitting the valve Va and the lower half of the circuit, an unbalanced modulator will be obtained.

The rectifiers shown in Figs. 1 and 4 may be diodes, or valves arranged for operation as rectifiers or they may be dry rectifiers of the copper oxide or selenium type. If a double diode-triode is used in the oscillator G, for example, the two diodes included therein may serve for X1 and X2 or for X3 andxi.

The electronic switch described in' Fig. 4 may be modified if it is desired to employ it in circumstances different from those of Fig. 1. Thus the anodes of the two valves could be separated and connected to different output circuits, whereby the incoming waves could be switched alternately to the two output circuits. Furthermore, the two cathodes could be energised in the same phase instead of in opposite phase, and/or with unequal voltages.

By omitting the valve V2 and its connections, then a simple open-an-close type of switch would be obtained whereby the waves could be intermittently supplied to a load circuit.

The applications of the electronic switch are not restricted to use with high frequency waves and it may be employed in any circuits requiring a change-over switch or an open-and-close switch, for example, in telegraph circuits.

The discriminating circuit D of Fig. 1 and the electronic switch of Fig. 4 may be combined together so as to share certain of the elements, whereby some economy is achieved. This arrangement is shown in Fig. 5, in which the elements which are the same as in Figs. 1 and 4 are given the same designations and will not be again described. It will be noted that the control grids of the valves V1 and V2 are alternately biassed from the transformer Tl. forming part of the discriminator D, so that an additional transformer (T3 in Fig. 4) is not really necessary. A further. simplification enables the rectifiers X3 and X4 and resistances R7 and Rs to be omitted. In this case the leads II and I2 will be connected to the terminals l and I8 instead of as shown to the terminals l3 and I4. The rectifiers X1 and X2 then serve the double purpose of rectifying the waves in the discriminator and limiting the positive swings of the control grids of the valves V1 and V2.

Fig. 6 shows a preferredform oi the electronic switch portion of Fig. 5 suitable for use at very short wavelengths (such as 90 cm., for example). The two valves V1 and V: are preferably of the disc seal type such for example as is described in the specification of the co-pending United States application No. 445,509, filed June 2, 1942 Patent No. 2,419,544 issued April 29, 1947 or No. 457,790 or 457,791, both filed September 9th, 1942. Patents 2,428,520 and 2,426,198 issued Sept. 30, 1947 and Aug. 26, 1947 respectively. They are mounted through apertures in a double partition ll dividing a metal box It into two compartments. The discs are sandwiched between the two parts of the partition with sheets of mica or like insulatin material (not shown) between, so that the discs close the apertures, but are insulated from the box. In this way the by-pass condensers K1 and K2 are formed.

The output resonant circuit L. C. of Fig. 5 is provided by the co-axial resonator formed between the central tube I9 and the inside of the box is, a trimming condenser C1 being provided. The anodes are coupled to the tube |9 through a blocking condenser H formed by an insulated collar surrounding the tube. The anode current is supplied through a conductor 20 passing through the tube I9 and a load resistance R11. The bypass condenser K3 is formed by a plate connected to the conductor 20 placed close to and insulated frm the box l8.

The transformer Z of Fig.5 comprises a loop formed by the two tubes 2| and 22 connected at one end to the box l8, and a loop 23 coupling therewith in the high current region. The loop 23 is connected between the two side aerials whose terminals are shown at A2 and A3, and the other 'ends of the tubes 2| and 22 are respectively connected directly to the cathodes of the valves. The heaters are supplied with current from the heating source HS through the resistances R9 and R and conductors 24 and 25 passing through the tubes 2| and 22. By-pass condensers K1 and K5 for the cathode heaters are formed by plates connected to the conductors 2i and 25. Trimming condensers C2 and C3 are provided for the two loops.

The terminals for the central aerial and the radio receiver are shown at Arand RR respectively, and are connected together by the conductor 26. The resistance R5 connects a suitable point on the tube 9 to the conductor. This point should preferably be such that the impedance shunting the direct connection between A1 and RR is high, so that the loss introduced into the communication channel is small. This will, of course, result in an inefficient connection to the radio receiver for the difference signals, but this is no objection, as only a small gain is required in the switch.

The conductors H and i 2 of Fig. 5 are connected to the valve discs as indicated in Fig. 6.v

The electronic switch shown in Fig. 6 is particularly suitable for very high frequencies and provides in addition amplification of the high frequency signal. It will be noted however that when no switching voltage is applied, both valves will give the same gain and since they are diflerentially connected at the input side, there will be no output. The modulated wave output is however obtained as soon as one of the valves is blocked.

While it will usually be preferable to tune the input and output circuits to the incoming waves, these circuits can if desired be also arranged so that the switch is not highly selective and will therefore work over a band of incoming frequencies without adjustment.

The use of grounded grid valves in t e electronic switch has several advantages. Thus, owing to the large amount of negative feedback inherent in an inverted amplifier, sta le operation of triode valves is maintained at ery high frequencies. For this reason, also. the gain will be substantially constant over a wide range of frequencies, even though resonant input and output circuits are used.

Furthermore, it is a characteristic of an inverted amplifier that its input resistance is very low, being approximately'equal to the reciprocal of the mutual conductance of the valve. Thus the input circuit is highly damped when the valve is operating. The feeders from the aerials A: and As may be coupled to the input circuit in such manner that correct matching is obtained when the valves are operating. Current will then flow in these aerials when a voltage is induced in them, so that they interfere with the antenna A1 which is thus rendered somewhat directional. This effect does not seriously aflect the operation of the direction finding device since the reference signal picked up by A1 will not vanish at any angle of arrival. It may however be desirable that the aerial A1 should be substantially non-directional when the direction finding device is not in use. This is achieved by making the transmission lines to the a'erials A2 and A3 equal in length to an integral number of half wavelength at the operating frequency. If this is done, when the valves V1 and V: are switched off the cathode-grid impedance becomes very high and th s high im= pedance is reflected back to the aerials A2 and A3 through thehalf wave transmission lines, and they practically cease to act as reflectors. This is one of the advantages of using inverted amplifiers, since it enables the aerials A: and As to be rendered ineffective by simply switching oiT the power to the valves V1 and V2. The above assumes tight coupling in the box with the input circuits of the valves. If the coupling is very loose, a phase change of 1r/2 will be introduced, and the lengths of the lines to the aerials A: and A: should be equal to an odd number of quarter wavelengths. The important point is that a very high impedance should be reflected to the aerials when the valves are switched off. Thus, in all cases, the total electrical path length from the aerials to the valve cathodes should be a whole number of half wavelengths.

What is claimed is:

1. An electric wave modulating arrangement comprising a thermionic valve having a cathode, a control grid and an anode, means for applying a high frequency alternating voltage wave between the said cathode and earth, means for maintaining the said control grid substantially at earth potential for the high frequency waves, means for applying a low frequency modulating signal voltage between the said control grid and earth, and means for deriving modulated high frequency waves from the said anode.

for waves at the given frequency,

rangement comprising two thermionic valves each having a cathode, a control grid and an anode,

means'for applying equal high frequency alternating voltage waves respectively in opposite phase between the said cathodes and earth, means for maintaining both the said control grids substantially at earth potential for the high frequency waves, means for applying equal low frequency modulating signal voltages respectively in opposite phase between the said control grids and earth, and means for deriving modulated high frequency waves from the said anodes.

3. An electronic switch for electric waves comprising a' thermionic valve having a cathode, a control grid and an anode, means for intermittently blocking the said valve, means ,for applying an alternating voltage at a given frequency between the said cathode and earth, means for maintaining the said control grid substantially at earth potential for waves at the given frequency, and means for deriving output waves from the said anode.

4. Ah electronic switch for electric waves comprising two thermionic valves each having a cathode, a control grid and an anode, means for intermittently blocking each of the said valves in alternate periods, means for applying alternating voltages at a given frequency between the said cathodes and earth, means @for maintaining both the said control grids substantially at earth potential and means for deriving output waves from the said anodes.

5. An electronic switch according to claim 4 inwhich the said blocking means comprises means for applying alternating biassing voltage waves at a frequency different from the given frequency in opposite phase between the said control grids respectively and earth, the amplitude of the said biassing waves being suflicient to bias the control grids beyond the cut-off point for the valves.

6. An electronic switch according to claim 22 in which the said limiting means comprises a pair of rectifiers respectively connected between the said control grids and earth and directed so as to conduct when the control grid potentials tend to become positive.

"I. An electronic switch according to claim 4 wherein said blocking means comprises means for applying frequency to the control grids respectively in opposite phase.

8. An electronic switch according to claim 4 in which the valves each have a disc sealed through the envelope thereof to serve as the terminal for the control grid.

9.An electronic switch according to claim 4, furthercomprising a metallic enclosure having a metallic partition, said valves each having a disc sealed through the envelope thereof to serve as the terminal for the control grid, said discs being mounted through a corresponding aperture in said metal partition dividing said metal box into two compartments, the said discs being arranged substantially to close the said aperture.

10. An electrical navigational direction indicating arrangement for a moving vehicle comprising a system of aerials adapted to derive from electrical waves collected thereby a differential voltage and a reference voltage, an electronic switch means for intermittently operating said switch comprising a source of switching signals, a radio receiver, and a discriminator, means for applying alternately the sum and the difference of the said reference and diiierential voltages to the alternating voltages at the given r the receiver comprising said switch, means for comparing the demodulated waves from the output of the receiver with the signals from the said switching source comprising said discriminator.

11. An arrangement according to claim 10 wherein said aerial system comprises three similar vertical aerials equally spaced on a line perpendicular to the direction of movement of the vehicle, and said switch comprises a pair oi electron discharge valves, means for applying the voltages derived from the two outer aerials differentially to the cathodes of the valves in the switch, means for connecting the anodes in parallel to the radio receiver, and means for applying the voltage derived from the central aerial simultaneously to the radio receiver in the same phase as the said difference voltage, or in the opposite phase.

12. An arrangement according to claim 16 wherein the outer aerials are connected to the said cathodes by transmission lines of such length that the equivalent electrical lengths of the total paths from the corresponding aerials to the cathodes of the switch valves is'a whole number of half wavelengths, and wherein the central aerial comprises a transmission line of length equal to an odd number of quarter wavelengths.

13. An arrangement according to claim 16 wherein the discriminator comprises a bridge circuit having rectifiers in adjacent arms.

14. An arrangement according to claim 16 wherein said discriminator comprises a bridge circuit, having rectifiers in adjacent arms, and said rectifiers in the said bridge circuit serve also for limiting the positive swings of the potentials of the control grids of the valves in the switch.

15. An electronic switch for electric waves comprising two thermionic valves each having a cathode, a control grid and an anode, means for intermittently blocking each of the said valves in alternate periods, means for applying alternating voltages at a given frequency between the said cathode and earth, means for maintaining both the said control grids substantially at earth potential for waves at the given frequency, means for deriving output waves from the said anode, a said blocking means comprising means for applying alternating biasing voltage waves at a frequency different from the given frequency in opposite phase between the said control grids respectively and earth, and means for limiting the positive swings of the said biasing waves in order to prevent grid current flow.

16. An electrical navigation indicating system for a. moving vehicle comprising a system of aerials adapted to derive from electrical waves collected thereby a difierential voltage and a reference voltage, an electronic switch comprising a pair of thermionic valves, said aerial system comprising three vertical aerials equally spaced and aligned perpendicular to the directions of the movement of the vehicle, means for applying the voltages derived from the two outer aerials differentially to the cathodes of said switch valves, a receiver coupled to said switch valves, means for intermittently blocking each of said switch valves in alternate periods comprising a source of switching signals, and means for applying the voltage derived from the central aerial simultaneously to said receiver in the same phase as the said differential voltage, or in the opposite phase, whereby the sum and difference of said reference and diiferential voltages are alternately applied to said receiver, means for comparing the demodulated waves from the output of said receiver with signals from the said switching signal source comprising a discriminator.

17. A radio navigation direction finder system comprising first radiant energy receiving antenna,

means for obtaining a difierential electrical signal indicative of the direction oi! radiant energy received second radiant energy receiving antenna means for obtaining a reference signal of said received energy, a receiver, a source of switching signals, means for alternately applying in phase opposition said differential signals to said receiver under the control of said switchingsignal source comprising switching means, means for applying the signals from said second antenna means in the same phase as said diflerential signals to said receiver, means for comparing the demodulated output of said receiver with the signals from said switching signal source, indicating means for translating said compared signals to provide indications of the direction of arrival of said received radiant energy. g

CHARLES EARP.

JEFFREY DENNIS WESTON.

REFERENCES CITED The following references are of record in the file of this patent: v

UNITED STATES PATENTS Number Name Date 1,868,945 Kruesi July 26, 1932 2,397,128 Cole et a1 Mar. 26, 1946 

